Theodore A. Fritz

Cusp energetic particle events: Implications for a major acceleration region of the magnetosphere

The Charge and Mass Magnetospheric Ion Composition Experiment (CAMMICE) on board the Polar spacecraft observed 75 energetic particle events in 1996 while the satellite was at apogee. All of these events were associated with a decrease in the magnitude of the local magnetic field measured by the Magnetic Field Experiment (MFE) on Polar. These new events showed several unusual features: (1) They were detected in the dayside polar cusp near the apogee of Polar with about 79% of the total events in the afternoonside and 21% in the morningside; (2) an individual event could last for hours; (3) the measured helium ion had energies up to and many times in excess of 2.4 MeV; (4) the intensity of 1-200 KeV/e helium was anticorrelated with the magnitude of the local geomagnetic field but correlated with the turbulent magnetic energy density; (5) the events were associated with an enhancement of the low-frequency magnetic noise, the spectrum of which typically extends from a few hertz to a few hundreds of hertz as measured by the Plasma Wave Instrument (PWI) on Polar; and (6) a seasonal variation was found for the occurrence rate of the events with a maximum in September. These characterized a new phenomenon which we are calling cusp energetic particle (CEP) events. The observed high charge state of helium and oxygen ions in the CEP events indicates a solar source for these particles. Furthermore, the measured 0.52-1.15 MeV helium flux was proportional to the difference between the maximum and the minimum magnetic field in the event. A possible explanation is that the energetic helium ions are energized from lower energy helium by a local acceleration mechanism associated with the high-altitude dayside cusp. These observations represent a potential discovery of a major acceleration region of the magnetosphere.

RAPID: The imaging energetic particle spectrometer on Cluster

The RAPID spectrometer (Research with Adaptive Particle Imaging Detectors) for the Cluster mission is an advanced particle detector for the analysis of suprathermal plasma distributions in the energy range from 20–400 keV for electrons, 40 keV–1500 keV (4000 keV) for hydrogen, and 10 keV nucl-1–1500 keV (4000 keV) for heavier ions. Novel detector concepts in combination with pin-hole acceptance allow the measurement of angular distributions over a range of 180° in polar angle for either species. Identification of the ionic component (particle mass A) is based on a two-dimensional analysis of the particles velocity and energy. Electrons are identified by the well-known energy-range relationship. Details of the detection techniques and in-orbit operations are described. Scientific objectives of this investigation are highlighted by the discussion of selected critical issues in geospace.

Electron Beams and Ion Composition Measured at Io and in Its Torus

Intense, magnetic field-aligned, bidirectional, energetic (>15 kiloelectron volts) electron beams were discovered by the Galileo energetic particles detector during the flyby of Io. These beams can carry sufficient energy flux into Jupiters atmosphere to produce a visible aurora at the footprint of the magnetic flux tube connecting Io to Jupiter. Composition measurements through the torus showed that the spatial distributions of protons, oxygen, and sulfur are different, with sulfur being the dominant energetic (>∼10 kiloelectron volts per nucleon) ion at closest approach.

The ISEE 1 and 2 Medium Energy Particles Experiment

We describe the medium energy particles experiment (MEPE) on-board ISEE 1 and 2. The MEPE consists of the WIM instrument on ISEE 1 and the KED instrument on ISEE 2. Both instruments employ solid-state detectors and magnetic analysis to measure the angular, energy, and intensity distributions of protons (ions) above 24 keV and electrons above 20 keV. The WIM instrument also includes a composition measurement employing ¿E by E and time-of-flight techniques. Three-parameter analysis is performed above 250 keV/nucleon, and single parameter analysis is performed above 125 keV/nucleon for helium through oxygen. Three-dimensional angular distributions are obtained through the use of a scan platform in the WIM instrument and multiple detector heads in the KED instrument. A variety of operational modes are used to optimize data collection from both instruments. Resolutions up to 128 channels in energy, 192 samples over the unit sphere in angle, and 0.095 s in time are available.

A new, temporarily confined population in the polar cap during the August 27, 1996 geomagnetic field distortion period

On August 27, 1996, a two-hour energetic heavy ion event (∼ 1 MeV) was detected at 8:25 UT at apogee (∼ 9 R e and an invariant latitude of ∼ 80°), by the Charge and Mass Magnetospheric Ion Composition Experiment onboard POLAR. The event, with a maximum spin averaged peak flux of ∼ 150 particles/(cm 2 -sr-s-MeV), showed three local peaks corresponding to three localized regions; the ion pitch angle distributions in the three regions were different from an isotropic distribution and different from each other. No comparable flux was observed by the WIND spacecraft. The appearance of lower energy He ++ and O >+2 during the event period indicates a solar source for these particles. From region 1 to 2 to 3, the helium energy spectra softened. A distorted magnetic field with three local minima corresponding to the three He peak fluxes was also observed by POLAR. A possible explanation is that the energetic He ions were energized from lower energy helium by a local acceleration mechanism that preferred smaller rigidity ions in the high altitude polar cusp region.

Correlation of cusp MeV helium with turbulent ULF power spectra and its implications

A new magnetospheric phenomenon called a cusp energetic particle (CEP) event was observed by the POLAR spacecraft. CEP events were detected in the dayside polar cusp near the apogee of POLAR and could last for hours, in which the measured helium ions had energies up to 8 MeV [Chen et al., 1997a, 1998]. A fundamental question is where do the cusp MeV ions come from? To answer this question, we have compared the ion flux in the CEP events with that in both the upstream and the downstream from the bow shock and found that bow shock acceleration cannot explain the measured ion flux in the CEP events. We have further determined the power spectra of the local magnetic field turbulence calculated over the September 18, 1996 CEP event periods for fluctuations in the ultra-low frequency (ULF) ranges, corresponding to periods of about 0.33–500 seconds. It is found that the integrated power of the turbulent spectra over the ULF ranges is correlated with the intensity of the MeV helium flux. These new results provides the first direct observation evidence that the high-altitude dayside cusp is a new acceleration region of the magnetosphere.

Numerical tracing of energetic particle drifts in a model magnetosphere

We present results from a model developed to study the motion of impulsively injected energetic particles which become trapped in the Earths magnetic field at geosynchronous orbit. The model is based on numerically solving the analytic expressions for the bounce average gradient and curvature drift in a model magnetic field. The predicted motion of trapped geosynchronous particles in the Tsyganenko and Usmanov (1982) model (TU-82) has characteristics which are not predicted by less sophisticated models. We investigate the motion of drifting particles predicted by the model under various conditions such as: Kp level, pitch angle, orientation of the magnetic field, and location of the origin of the drift shell. As a test of the predictions of the model, we apply it to observations of a substorm injection event which took place on October 16, 1983, and was observed by three geosynchronous satellites. The injection region for this event is found to span approximately 45° from near midnight to near 2100 LT. We also report the existence of a “periphery” outside the central injection where either injected ions or electrons, but not both, are observed with dispersionless signatures. The internal consistency of the predictions of the structure of the injection region using remote observations from the three geostationary satellites and the TU-82 field model is found to be quite good.

The discovery of trapped energetic electrons in the outer cusp

We report on the POLAR/ CEPPAD discovery of a trapped, 60°<θ<120° pitch angle electron population in the outer cusp (7-9+ Re), whose energetic electron component extends from below 30 keV to ∼2 MeV. Because the time variability in the outer cusp precludes mapping with POLAR, we have carried out test particle simulations using the Tsyganenko 1996 model (T96) to demonstrate the trapping of these energy electrons in the outer cusp region and the resonant frequencies of its trapped motion. We discuss the boundaries and regions of the cusp trap and show that it is analogous to the dipole trap. We show that the phase space densities observed there are equal or greater than the phase space densities observed in the radiation belts at constant magnetic moment, thus allowing the possibility of diffusive filling of the radiation belts from the cusp.

A quiescent state of 3 to 8 MeV radiation belt electrons

During a ∼3 month period in mid-1996 outer radiation belt electrons in the energy range from ∼ 3 to 8 MeV were diffusing inward and decaying in intensity with no internal or external source. Measurements from the HIST instrument on POLAR are used to constrain a model for time dependent lossy radial diffusion of these electrons, and to obtain estimates of a parameterized radial diffusion coefficient and lifetime. For lower energy electrons, of ∼ 1 to 3 MeV, a source at L > 6 is apparent throughout most of the same period.

Energetic particle signatures at Ganymede: Implications for Ganymede's magnetic field

The second encounter of the Galileo satellite with the Galilean moon Ganymede provided energetic particle measurements showing effects due to the presence of that moon. Jovian corotation signatures, present on approach to and departure from the Ganymede system, suddenly become much smaller when Galileo enters what has been termed Ganymedes magnetosphere. The location of these transitions agrees with magnetopause crossings identified by the magnetometer and plasma wave instruments. In Ganymedes magnetosphere, energetic ion and electron distributions display loss cone signatures whenever the Energetic Particles Detector (EPD) views along the magnetic field line. The loss cone measurements are used to estimate Ganymedes surface magnetic field along the satellite track. The results agree with model projections to Ganymedes polar cap and support the existence of a Ganymede-intrinsic magnetic field. An evolution from single to double loss cone also occurs with increasing electron energy.